Practising Law Institute, a prestigious nonprofit continuing legal education organization, will soon have new digs on booming Sixth Avenue.

The Institute just signed a lease for 68,000 square feet at Silverstein-managed 1177 Sixth, aka the Americas Tower, at 45th Street — a modest expansion over the space it’ll leave behind at SL Green’s 810 Seventh Ave., sources said.

PLI offers courses and other support resources to the legal profession. The asking rent was $66 a square foot.

A Jones Lang LaSalle team of Paul Glickman, Frank Doyle, Mitti Liebersohn and Cynthia Wasserberger repped Silverstein Properties. Glickman said PLI will have its own entrance on West 45th Street.

Glickman cited the tower’s “tremendous infrastructure, large and efficient floor plates and lobby enhancements which are in progress” as helping lure PLI.

PLI was repped by Studley’s Ira Schulman, David Goldstein, Patrick Gardner and Howard Poretsky, who declined to comment.

***

When the city gets around to making Vanderbilt Avenue a “pedestrian mall,” don’t expect a remake of Times Square.

The rethinking of Vanderbilt is coming from the Department of City Planning, not the Department of Transportation.

Although plans are far from complete, it sounds like Planning Commissioner Amanda Burden has no taste for another cheaply paved plaza with temporary furniture. She told us the idea is to “bring elegance and stature befitting the distinction of the location.”

Fears that cars won’t be able to use the avenue at all similarly seem groundless. Burden said cars going up Madison Avenue will turn right on East 44th, then right again south on Vanderbilt to pick up passengers at Grand Central Terminal, and exit via 43rd Street.

Problem solved.

Burden & Co. are also fully aware of the needs of companies like Bank of America and JPMorgan Chase, which have entrances on Vanderbilt — although how they’ll be addressed remains to be seen.

The plan might take a while to happen. Sources said the city hopes to pay for its long-term redesign out of District Improvement Bonus funds, to which developers will contribute for floor area ratio (FAR) bonuses under proposed Grand Central area rezoning — which won’t take effect until 2017.

***

A Florida congressman has called out the General Services Administration for signing its lease at 1 World Trade Center without his blessing — and is threatening unspecified “potential next steps.”

Rep. John Mica, whose House Committee on Transportation and Infrastructure oversees GSA spending, is seething that the agency inked the 20-year, 270,000 square-foot lease after years of delays.

Mica’s rep told us yesterday, “We want to hear GSA’s justification for their actions and will take that into consideration for any potential next steps.”

That followed a July 25 letter from Mica and two members of his panel to GSA acting administrator Daniel Tangherlini.

The letter, seen by Realty Check, expressed “deep concern and disappointment” over the lease signing, which it says was done “without an approved prospectus as required.”

The GSA responded yesterday: “Although GSA has the legal authority to proceed, we chose to send the lease to Congress in the spirit of cooperation and transparency. . . . The House committee had ample opportunity to review and act, and it did not.”

Sen. Chuck Schumer, who prodded the agency to get the deal done, told us, “The GSA had the authority to move forward with the lease, and they did. It was far past time that it got done.”

But Mica has stalled on other GSA leases. Three days ago, he said he was holding the agency’s “feet to the fire” and “investigating outrageous abuses of taxpayers’ money.”

The 1 WTC lease is much smaller than Condé Nast’s 1.1 million feet, but allowed the Port Authority and Douglas Durst to crow that their tower, to open in early 2015, is already more than half full.

The GSA, after many delays, in the spring dotted the “i”s and crossed the “t”s on the deal, which was approved by the PA, Durst and a Senate committee.

The 20-year deal calls for an average “face” rent of $66.33 per square foot over the first five years and a “maximum proposed” rent of $81.91 per square foot in years 16-20. But the GSA will effectively pay much less, due to subsidies and a tenant improvement allowance.